1. Field of the Invention
This invention relates generally to new and improved methods and apparatus for directionally drilling a borehole into the earth, and particularly to a directional drilling system where the weight being applied to a rotary drill bit having asymmetric cutters is increased in a synchronous manner during each revolution to cause the bit to drill preferentially in a certain azimuthal direction.
2. Description of the Related Art
Various techniques have been used to drill boreholes directionally toward a designated underground target earth formation. As used herein unless the context indicates otherwise, the word "directional" means the inclination of a borehole with respect to vertical, and the azimuth of such inclination with respect to magnetic North. A well bore drilled from an offshore platform, for example, might have an initial section that extends substantially vertical to a given depth, and there the borehole is curved at a certain azimuth by gradually building up the inclination. Then the borehole may be drilled straight ahead in that direction until the hole bottom approaches a particular target, at which point the borehole may be curved gradually back downward to the vertical while holding the same azimuth. Finally the borehole is drilled straight ahead, i.e. vertically downward, through the target earth formation. In this manner a large number of wells which penetrate the formation at numerous spaced points can be drilled from a single platform in order to drain the formation of oil and/or gas in an efficient and economic manner.
Various devices have been employed to achieve directional drilling as set forth above. One system provides a drill string having stabilizers positioned thereon at certain distances so as to achieve directional drilling using the pendulum effect of the lower section of the drill string. This system has the disadvantage that the drilling string must be withdrawn from the well several times during the drilling to change the number and location of stabilizers. Of course each round trip is time-consuming and expensive. Another system uses a downhole motor to drive the drill bit, together with a bent sub located in the drill string above the motor. The bent sub provides an angular offset that can be used to orient the bit in the desired azimuth, particularly where a directional measurement system in included in the drill string. While adequate to drill a curved borehole, this system is not capable of drilling a straight or tangent hole section. Thus the drill string must be tripped out to remove the bent sub when a straight hole section is to be drilled.
Still another directional drilling system uses a "steerable" drilling motor where the bend angle is provided in a housing between the motor power section and the bit. The bent housing causes the bit to drill along a curved path and substantially reduces the stresses in the threaded connections which support the bend. When straight ahead drilling is needed, the drill string is rotated at the surface so that such rotation is superimposed over that of the motor drive shaft. This causes the bend point to merely orbit about the axis of the borehole so that the bit drills straight ahead rather than along a curve. To resume directional drilling the superimposed rotation is stopped. Although this type of directional drilling is effective and has been widely used, the drilling motor is a specialized and expensive item of equipment that tends to wear out somewhat rapidly.
An object of the present invention is to provide a new and improved directional drilling method and system which avoids the difficulties and problems experienced with the foregoing prior systems.
Another object of the present invention is to provide a new and improved directional drilling tool where additional weight is periodically and synchronously applied to an asymmetric rotary drill bit to cause the bit to drill along a curved path.
Another object of the present invention is to provide a new and improved directional drilling system where a rotary drill bit having asymmetrically arranged cutters is subjected to increased weight during a selected portion of each revolution so that the bit drills preferentially on one side of the bottom of the borehole and causes the hole to be drilled along a curve in a selected azimuthal direction.
Still another object of the present invention is to provide a method for controlling the weight-on-bit downhole with minimal intervention from the surface.